1. Field of the Invention
The present invention relates to an optical scanning apparatus for forming an electrostatic latent image on a photosensitive member by a light beam scanned by a rotational polygon mirror, and an image forming apparatus including the optical scanning apparatus.
2. Description of the Related Art
An electrophotographic image forming apparatus forms an image based on the following process. A light beam output from a light source, such as a semiconductor laser, based on input image data is deflected by a deflect and scan apparatus, such as a rotational polygon mirror (hereinafter, “polygon mirror”) and a galvanometer mirror, and is converted into scanning light. A photosensitive member having a uniformly charged surface is scanned by the light beam to form an electrostatic latent image on the photosensitive member.
The electrostatic latent image is developed by a toner, and the developed toner image is transferred onto a sheet of recording paper. Then, the toner image on the recording paper is heat-fixed to form an image on the recording paper.
The polygon mirror and light source are included in the optical scanning apparatus, which is attached to the image forming apparatus. FIG. 9 illustrates an example of a configuration of an optical scanning apparatus. During image formation, a polygon mirror 901 is rotationally driven at a predetermined rotation speed (rotation number) by a drive motor 902.
From a light source 903, a light beam is output based on an image signal modulated based on image data. This light beam is incident onto a reflection surface of the polygon mirror 901. The light beam incident on the reflection surface of the polygon mirror 901 is reflected by the reflection surface of the rotating polygon mirror 901, thereby becoming scanning light. This scanning light passes through an image-forming optical system 904, and is focused on a photosensitive member 906.
As illustrated in FIG. 9, the optical scanning apparatus includes a beam detector (BD) 905 which receives the light beam scanned by the polygon mirror 901. The BD 905 is a sensor provided to synchronize the image writing start position for each scan.
A central processing unit (CPU) (not illustrated) controls the output of the light beam from the light source 903 at a timing based on a synchronization signal (also referred herein as a “BD signal”) generated by the light beam incident on the BD 905. Further, the CPU controls the drive motor 902 so that the cycle of this synchronization signal is a constant cycle.
Further, the CPU causes the light source 903 to emit light for each scan. These light beams are detected by a detection unit such as a photodiode. Based on the detection result, the CPU controls the drive current applied to the light source so that the light quantity of the light beams output from the light source when forming the electrostatic latent image becomes a predetermined light quantity (auto power control, hereinafter “APC”). By performing APC, fluctuation in the image density due to fluctuation in the light beam light quantity is suppressed.
Conventionally, in an image forming apparatus having a two-sided printing function for forming an image on both sides of a sheet of recording paper, there has been the problem that the size of the image on the front and back has been different. When forming an image on one side of the recording paper (hereinafter, “front side”), the recording paper passes through a fixing device. By passing the recording paper through the fixing device, moisture absorbed in the recording paper evaporates.
Since the moisture content thus decreases, the size of the recording paper shrinks. Consequently, the size of the image formed on the front side of the recording paper also shrinks. When forming an image on the back side of the thus-shrunk recording paper, unless the size of the image to be formed on the back side is also shrunk, the size of the image to be formed on the back side will be larger than the size of the image formed on the front side. As a result, images having a different size on the front and the back of the recording paper are formed.
To resolve this problem, Japanese Patent Application Laid-Open No. 2007-236031 proposes a technique in which, when performing two-sided printing, the sizes of the image on the front and back are matched by adjusting the image magnification of one side during image formation.
For example, to make the size of the image to be formed on the back side smaller than the size of the image on the front side, when forming the image on the back side, the cycle of the image clock is made shorter and the rotation speed of the polygon mirror is made quicker by a predetermined ratio than a predetermined rotation speed with respect to the front side, than when forming the image on the front side. By controlling in such a manner, the size of the image formed on the back side can be shrunk more than the size of the image formed on the front side.
Further, when a recording medium which has not passed through the fixing device immediately after the images are formed on the front and back of a predetermined recording medium is conveyed, the rotation speed of the polygon mirror needs to be returned to a predetermined rotation speed.
Further, as discussed in Japanese Patent Application Laid-Open No. 05-208522, while continuously forming images on a plurality of recording media when changing the resolution midway through forming the images, it is necessary to control the rotation speed to a desired speed by accelerating or decelerating the rotation speed of the polygon mirror.
However, in some cases the synchronization signal which should be generated during each scan, when the rotation speed of the polygon mirror is changed, is not generated. An example will now be described with reference to FIG. 10B in which the drive motor 902 is accelerated in a situation in which synchronization signals are not being generated in an image forming apparatus capable of forming a plurality of scanning lines during each scan by using a plurality of light emitting elements as the light source.
More specifically, FIG. 10B illustrates a laser control state and a timing for generating a synchronization signal for each scan of an optical scanning apparatus having eight light emitting elements (light emitting elements A to H) outputting a light beam.
Point (1) in FIG. 10B indicates the light emission timing of each light emitting element. Point (2) in FIG. 10B indicates the timing when a BD signal is generated by causing the light emitting element A to emit light when the rotation speed of the polygon mirror 901 is set at a steady speed (100%).
Point (3) in FIG. 10B indicates the timing when a BD signal is generated by causing the light emitting element A to emit light when the rotation speed of the polygon mirror 901 is set at 1% acceleration (101% speed).
As indicated by point (1) in FIG. 10B, during the image region scanning period, a light beam is output from each light emitting element based on the image clock and input image data. The expression “during the image region scanning period” refers to the period during which laser light output from the light source based on the input image data scans the photosensitive member.
During the non-image region scanning period after the image region scanning period, the CPU temporarily turns off all of the light emitting elements, and then sequentially causes the light emitting elements B to H to emit light. Based on a detection result of the light beam output from each of the light emitting elements, APC is performed for the light emitting elements B to H.
Further, the CPU causes a light beam to be output from the light emitting element A. The CPU supplies a drive current to the light emitting element A so that a light beam is output at a timing before the light beam output from the light emitting element A passes the BD 905. The CPU performs APC for the light emitting element A based on a detection result from a photosensor.
Subsequently, the laser light beam output from the light emitting element A due to the CPU keeping the light emitting element A turned on is incident on the BD 905. Consequently, the BD signal is generated.
Then, the CPU causes a light beam to be output from each light emitting element, based on the generated timing of the BD signal during a subsequent period of scanning an image forming region, and based on the image data. To each of the light emitting elements at this stage, a drive current set due to performing APC is supplied. Consequently, a light beam having a predetermined light quantity is output from each light emitting element.
When accelerating the rotation speed of the polygon mirror from the steady speed 100% to a rotation speed of 101%, as illustrated in FIG. 10A, a rotation speed overshoot occurs, and the rotation speed temporarily reaches a speed of more than 101%. At this stage, as indicated by point (4) in FIG. 10B, to generate the BD signal, the light emitting element A has to be turned on at a timing before that of the timing at which the light emitting element A indicated by points (2) and (3) in FIG. 10B is turned on.
However, since there are periods for performing APC for other light emitting elements, the turn-on timing of the light emitting element A cannot be brought forward. In such a case, since the light beam output from the light emitting element A won't be incident onto the BD, the BD signal is not generated. If the BD signal is not generated, the CPU will determine that the cycle of the BD signal has become longer.
Consequently, the CPU controls the drive motor 902 to increase the rotation speed of the polygon mirror 901 in order to shorten the cycle of the BD signal. Despite the fact that the polygon mirror 901 is rotating nearly at the target rotation speed, a large acceleration control is applied. Consequently, it takes time to converge the rotation speed of the polygon mirror 901 to the target rotation speed.
Further, if the rotation speed of the polygon mirror is decelerated, in some cases the synchronization signal cannot be generated unless the light emitting element A is turned on during the unlit period immediately after the turn-on period of the light emitting element A indicated by point (1) in FIG. 10B due to undershooting of the rotation speed.
Thus, when the rotation speed of the polygon mirror is changed, in some cases the synchronization signal cannot be generated due to overshooting or undershooting of the rotation speed.